High frequency coupling transformer



May'2l, 1935. K. POSTHUMUS' HIGH FREQUENCY COUPLING TRANSFORMER Filed Sept. l6, 1.950

W E R INVENTOR KLAAS POSTHUMUS ATTORNEY Patented May 21, 1935 UNITED STATES HIGH FREQUENCY COUPLING TRANSFORMER Klaas Post humus, Eindhoven, Netherlands, assignor to Radio Corporation of America, a corporation of Delaware Application September 16, 1930, Serial No. 482,196 In the Netherlands October 25, 1929 6 Claims. (01. 178-44) Theinvention relates to high frequency transformers, and it has for its main object to provide such a construction thereof that a characteristic is obtained which has a smooth course on a broad band of frequencies.

It is well known that an inductance coil should not exclusively be considered to be the carrier of a concentrated self-inductance, but that, more particularly with high frequencies, the fact must be taken into account that the coil'has a divided inductance and capacity, in consequence of which the current intensity in the coil has not the same phase over the entire length. It is commonly known that an inductance coil has a natural wave-length which is determined by the divided inductance and capacity. In this case the inductance coil may be regarded as a line on which, due to reflection at the ends, stationary waves are produced. The presence of stationary waves can be easily demonstrated on a so-called Tesla-coil.

It is evident that if on the coil occurs, for example, a whole wave-length, the inducing eflect on a second coil is substantially equal to zero. If

the coil is connected in such manner that at the ends voltage nodes are produced, the coil when containing a half-wave length, will-practically have no longer an inducing effect on a second coil, which constitutes for example, the secondary winding of a transformer whose primary winding is formed by the first-mentioned coil. When the frequency at which the primary coil is excited, increases so that more than one half-wave length lies on it, a tension will be induced again in the secondary winding. It is evident, however, that whenever the coil contains a whole number of half wave lengths the secondary winding is'practically inactive.

According to the invention, in order to increase the frequency range for which the transformer is active, an ohmic resistance is connected in series with the primary winding, said resistance having a value equal to the square root of the quotient of inductance and capacity of the unit of length,

serving for the-supply of a great current intenin other words equal to the surge impedance of the coil regarded as a transmission line.

If for the secondary winding use is made of a coil which has only a slight number of turns and whose natural wave-length is very small, it will be possible to obtain a great-current intensity .for all frequencies lying beneath the frequency at which the primary winding contains a whole wave-length. In fact,-byconnecting in series the said resistance the current in the primary winding of the transformer is made a current having the shape of an advancing wave; consequently, a

'make the secondary winding a transmission line which is closed through a resistance equal to the surge impedance. Forexample, at a plurality of points the secondary winding may be connected to capacities whose other ends are connected to a common conductor, the ends of the secondary windingbeing each connected to the same common conductor through a resistance whose value is equal to thesurge impedance. If desired, one of the resistances may be an arbitrary one; it may be given, for example, the value of zero orinfinity.

According to the invention, there may be provided, in addition, a screen of conducting material which may be regarded as the second con-' ductor of the transmission line of which the coil constitutes the first conductor. This screen must be constructed so as to comprise no closed parts which enclose parts of the magnetic flux; it may be provided, for example with longitudinal slits.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect. 1 l

v The invention will be explained more fully with reference to theaccompanying drawing which represents, by way of example, some embodiments thereof. 1

Fig. 1 represents a high frequency transformer sity.

Fig. 2 shows a Fig. -1.

fig. 3 represents a transformerin which both in the primary and in the secondary winding the current ismade'a current having the shape of an advancing wave.

Referring to Fig. 1, l is 'a triode while L1 denotes the primary winding of a transformer. In series'with this winding is connected a resistance R which is connected to the earth A. In the figmodified mode of execution of ures, 0 denotes the capacities of the various parts of the winding L1 with respect to the earth. The primary winding may consequently be regarded as a transmission line with a divided capacity and inductance. If the capacity and the inductance per unit of length are called C and L respectively, the surge impedance of the line is equal to If the resistance R is made equal to the surge impedance, the primary winding will behave as a transmission line of infinite length with a' surge impedance R. In Fig. l the secondary winding is denoted by L2. Suppose the number of turns of the winding L2 is much smaller than that of the winding L1. In this case L2 may be consid-- ered to be a concentrated inductance. So long as the electrical length of the primary winding is smaller than one wave length, a tension'will be induced in the; winding L2. When the electrical length becomes equal to the wave length, the total electromotive force induced in L2 will be equal to zero. Without resistance R, however, the induced electromotive force would already be equal to zero when the length of the primary Winding is equal to a half-wave-length. The arrangement shown in Fig. 1 consequently renders it possible totransmit a broader range of fre quencies. V

1 In Fig. 2 is provided a screen S which may be regarded as the second conductor of the line. This screen may be arranged either inside or outside the primary winding, but care should of course be taken to prevent it from acting as a short-circuiting ring for the primary winding. For this purpose, the screen is provided with 1011- gitudinal slits. and, besides, it comprises no parts which, possess uninterrupted conducting portions enclosing the field of the primary coil.

In Fig. 3 the secondary winding L2 too, is executed as a transmission line. For this purpose, artificial capacities c are provided at difierent points of the secondary winding, said capacities being connected on the other side to a common conductor 2. The ends of the secondary winding are connected to this conductor through resistances R. It is evident that the ends of the resistances B may also be connected to one another, in which event the ends of the capacities which are not connected to the coil are united in the same point. The secondary tension may be tapped from one of the resistances R. The resistances R are made equal again to the surge impedance of the coil.

. The transformer according to Fig. 3 functions as follows: In the coil L1 flows a current having the shape of an advancing wave. When considering two quite opposite points of the primary and the secondary coil, for example the points 3 and 4, the point 3 will set up in point 5 an electromotive force which produces a current which is propagated from 4 in two directions. If in Fig. 3 the direction in which the current is propagated is that of the arrow, in the resistance R at the end 5 of the secondary coil, will be set up a current which is proportional to the frequency whereas in the resistance R atthe end 6 of the secondary coil occurs a current which does not smoothly vary with the frequency, said current showing a rapid succession of maxima and minima. For this reason the tension is taken off from the resistance R at the end 5 of the coil.

Even if the speed of propagation along the secondary winding is not equal to that along the primary winding, an improvement will be obtained with respect to the existing transformers but in this case at neither of both ends the current is independent of the frequency. Since the number of artificial capacities c is always limited, it is of course practically impossible to obtain a perfect independency'of the frequency, but it has been found-that even with a relatively limited number of capacities c the frequency dependency is only slight.

Inthe following manner one may form an idea of the phenomena occurring in the resistances R at the ends of the secondary winding of the transformer .of Fig. 3; The current in the primary winding has the shape of an advancing wave and is propagated in one direction only. In the secondary winding is consequently induced an electromotive force which also has the shape of an advancing wave and is propagated in one direction only.

Provided the coil length of the windings is the same and equal to h, if -01 is the speed of propagation along the primary coil, 02 that along the secondary. coil, E the amplitude of the induced electromotive force in the secondary winding, (0 the frequency and if .r is taken as the co-ordinate advancing along the coils, theinduced electromotive force in an element do: of the secondary winding maybe represented as:

iw t wherein e=the base of the natural system of logarithms and '1'= 1/I Herein it is assumed that the current in an element of'the primary winding only influences the element of the secondary winding which has the same co-ordinate :13. This electromotive force produces two currents I1 and I2 in the secondary winding which are propagated in opposite directions at a speed 02 and whose amplitude may be assumed to be aE.

In order to calculate the resulting current intensity at the end h we proceed as follows:

The current I2 coming from the element'da: and which is transmitted to the end of it arrives at that end an interval current intensity in h at the instant t consequently is: I

also an alternating current with constant amplitude. 7. As E is produced by magnetic induction, E will and therefore if be proportional to so that the amplitude of the current in the resistance R at the lower end of the winding is proportional to the frequency.

For the upper end of the secondary winding :r=0 we will find for the current at the instant t coming from the element do:

The amplitude consequently shows maxima for the frequencies and minima for x=0,l,2,3, etc.

=g'rrx, x=0,1,2,3, etc.

The production of maxima and minima at a:=h is only perfectly suppressed when the speed of propagation along the secondary winding is equal to that along the primary winding.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications in the circuit arrangements, as well as in the apparatus employed, may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In an electrical transmission system, a source of high frequency energy, a load circuit, and a transformer coupling said source andlo'ad, a resistor in series with the primary winding of the transformer, said resistor having a value equal to the surge impedance of the primary winding regarded as a transmission line, and the secondary winding of the transformer being constructed to be resonant at a relatively small natural wave-length, said transformer thereby being adapted to transmit with relatively great current intensity all frequencies below a frequency at which said primary winding has an electrical length of one wave length, and a resistor having a value equal to the surge impedance of the secondary winding connected in series with said secondary winding at each end thereof.

2. In an electrical transmission system, a source of high frequency energy, a load circuit, and a transformer coupling said source and load, a resistor in series with the primary winding of the transformer, said resistor having a value equal to the surge impedance of the primary winding regarded as a transmission line, and the secondary winding of the transformer being constructed to be resonant at a relatively small natural wave-length, said transformer thereby being adapted to transmit with relatively great current intensity all frequencies below a frequency at which said primary winding has an electrical length of one wave length, a resistance connected in series with said secondary winding, said resistance having a value equal to the surge impedance of said secondary winding and output leads connected across said resistance, said source comprising an electron discharge tube having its output electrodes connected across said primary winding;

3. In combination, a vacuum tube having an anode a cathode and agrid, means for applying waves to be amplified across the grid and cathode, a coil having one of its terminals connected to said anode, a resistance equal in value to the surge impedance of said coil having one of its terminals connected to the free terminal of said coil, means connecting the other terminal of said resistance to said cathode, and, an output circuit coupled to said coil.

4. Apparatus as claimed in the preceding claim characterized by the fact that the end of said resistance removed from said coil is connected directly to ground and being further characterized by the fact that a source of anode potential is connected between said grounding connection and said cathode.

5. Apparatus as claimed in claim 3 characterized by the fact that an electrostatic screen is placed in close proximity to said coil and being further characterized by the fact that said screen is connected to a point on said resistor removed from said coil.

6. In combination, a multi-electrode tube having an anode a cathode and a grid, means for applying input waves across said grid and cathode, the series combination of a coil, a resistance and a source of potential connected between said anode and cathode, said coil being connected directly to said plate, said resistance having a value substantially equal to the surge impedance of said coil being connected directly to said coil and said source of potential being connected between said resistance and said cathode, means grounding the connection between said source of potential and said resistance, an electrostatic shield arranged in close proximity to said coil, means grounding said shield, and, an output circuit coupled to said coil. 

